Laser illuminated projection displays

ABSTRACT

A projection video display includes at least one laser for delivering a light beam. The display includes a beam homogenizer and a condenser lens. A scanning arrangement is provided for scanning the light in beam in a particular pattern over the condenser lens in a manner that effectively increases the beam divergence. The scanned beam is homogenized by a beam homogenizer and a spatial light modulator is arranged to receive the homogenized scanned light beam and spatially modulate the beam in accordance with a component of an image to be displayed. Projection optics are projecting the homogenized scanned light beam onto a screen. The scanning provides that the homogenized scanned light beam at the screen has a coherence radius less than the original coherence radius of the beam. The reduced coherence radius contributes to minimizing speckle contrast in the image displayed on the screen. The screen includes one or more features providing a further contribution to minimizing speckle contrast in the displayed image. In one example, the screen includes a transparent cell containing a liquid having light scattering particles in suspension.

TECHNICAL FIELD OF THE INVENTION

The present invention relates in general to video displays, particularlyprojection TV displays, illuminated by laser radiation. The inventionrelates in particular to reducing speckle effects in such displays.

DISCUSSION OF BACKGROUND ART

Several proposed arrangements of laser light illuminated projectiondisplays have been described in many prior-art documents. It isbelieved, however, that, despite these prior-art descriptions there hasnot yet been produced a commercially available, consumer market,laser-illuminated video display of any kind.

In order for such a display to be acceptable in a consumer electronicmarket, such a display would need to be competitive in cost andtechnical performance with conventional projection displays that areilluminated with a white light source such as a mercury lamp.Requirements for a laser used in a laser projection display include,reliability, compactness, energy efficiency, closeness of the laseroutput wavelength to an additive primary (r, g, or b) wavelength, and abeam quality compatible with spatial light modulators used in thedisplay. Perhaps equally important, the laser should be inexpensive andeasy to manufacture in volumes that will be required in a consumermarket. Further, the display must include measures to eliminateperceivable “speckle” and other effects in the display resulting fromcoherence of the laser radiation. These requirements are addressed inembodiments of an inventive laser display described hereinbelow.

SUMMARY OF THE INVENTION

In one aspect, the present invention is directed to a projection displaycomprising at least one laser for delivering a light beam. The lightbeam has an original beam divergence and an original coherence radius.The display includes a beam homogenizer and a condenser lens. A scanningarrangement is provided for scanning the light in beam in a particularpattern over the condenser lens lens, such that, averaged over a timeless than about the response time of a human eye, the light beam has aneffective beam divergence greater than the original divergence thereof.The scanned beam is delivered from the condenser lens into the beamhomogenizer to be homogenized thereby. A spatial light modulator isarranged to receive the homogenized scanned light beam in accordancewith a component of an image to be displayed. A screen is provided fordisplaying the image. Projection optics are provided for projecting thehomogenized scanned light beam onto the screen. The homogenized scannedlight beam at the screen has a reduced coherence radius less than theoriginal coherence radius thereof as a result of the scanning thereof.The reduced coherence radius provides a contribution to minimizingspeckle contrast in the image displayed on the screen. The screenincludes one or more features for provides a further contribution tominimizing speckle contrast in the image displayed thereon.

In one embodiment of the display, the screen includes said screenincludes a first transparent sheet having a plurality of raised featuresdistributed over a surface thereof. Each of the features has a dimensiongreater than the reduced coherence radius of the light beam, but lessthan a dimension that would be resolvable by a normal human eye at anormal viewing distance from the screen.

In another embodiment of the display the screen includes a transparentcell containing a transparent fluid having particles therein. Theparticles have a dimension sufficiently small to scatter light. In apreferred example, the screen includes an agitating arrangement operablefor causing the particles to be in random motion with each other.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, schematically illustrate a preferredembodiment of the present invention, and together with the generaldescription given above and the detailed description of the preferredembodiment given below, serve to explain the principles of the presentinvention.

FIG. 1 is an elevation view, partially in functional block diagram form,schematically illustrating a first preferred embodiment of alaser-illuminated, rear projection television RPTV) display inaccordance with the present invention, including a two-arm foldedresonator, frequency-doubled, optically pumped semiconductor laserpumped by a light from a diode-laser array and providing outputradiation in multiple transverse modes, a video system including timeand spatial modulators for second-harmonic radiation delivered by thelaser resonator, and projection optics for projecting temporally andspatially modulated light onto a screen to provide the display.

FIG. 2 is a graph schematically illustrating computed spot size of theTEM₀₀ mode at two different positions in the laser resonator of FIG. 1as a function of length of one-arm of the resonator.

FIG. 3 is a graph schematically illustrating measured divergence of afrequency-doubled output beam in one example of the laser resonator ofFIG. 1, in two mutually perpendicular axes as a function of length ofone arm of the resonator.

FIG. 4 is a graph schematically illustrating total second-harmonicoutput power as a function of beam divergence in the resonator exampleof FIG. 3.

FIG. 5 is an elevation view, partially in functional block diagram form,schematically illustrating a second preferred embodiment of alaser-illuminated, rear projection television display in accordance withthe present invention, similar to the display of FIG. 1, but whereintime modulation of the second harmonic output radiation of the laserresonator is provided by modulating the pump-light output of thediode-laser array under control of the video system.

FIG. 5A is a graph schematically illustrating peak frequency-doubledoutput power as a function of peak pump-light power for amultiple-transverse-mode OPS-laser in accordance with the presentinvention in CW operation and in directly modulated operation.

FIG. 6 is an elevation view, partially in functional block diagram form,schematically illustrating a display similar to the display of FIG. 1,but further including a two-axis scanning arrangement including tworotating, multi-faceted scanner wheels having a lens therebetween, thearrangement for providing an apparently high divergence in a combinedlaser beam delivered to the video system.

FIG. 6A is a plan view from above, schematically illustrating furtherdetails of the scanner wheels and lens of the scanning arrangement ofFIG. 6.

FIG. 7 is an elevation view, schematically illustrating one alternativescanning arrangement for providing an apparently high divergence in acombined laser beam delivered to a video system in accordance with thepresent invention, the arrangement including a rotating wedge-shapedoptical element.

FIG. 8 is an elevation view, schematically illustrating anotheralternative scanning arrangement for providing an apparently highdivergence in a combined laser beam delivered to a video system inaccordance with the present invention, the arrangement including astationary mirror, and a rotating mirror tilted on the rotation axisthereof.

FIG. 9 is an elevation view, schematically illustrating yet anotheralternative scanning arrangement for providing an apparently highdivergence in a combined laser beam delivered to a video system inaccordance with the present invention, the arrangement including tworotating mirrors each thereof tilted on the rotation axis thereof.

FIG. 10 is an elevation view, schematically illustrating still anotheralternative scanning arrangement for providing an apparently highdivergence in a combined laser beam delivered to a video system inaccordance with the present invention, the arrangement including a pairof galvanometer mirrors arranged to scan an incident combined laser beamin two mutually perpendicular axes.

FIG. 11 is an elevation view, schematically illustrating a furtheralternative scanning arrangement for providing an apparently highdivergence in a combined laser beam delivered to a video system inaccordance with the present invention, the arrangement including asingle, rotatable, faceted, scanner wheel having facets thereofprogressively tilted with respect to the rotation axis of the wheel andarranged to scan an incident combined laser beam in two mutuallyperpendicular axes.

FIG. 11A is a plan view from above, schematically illustrating furtherdetails of the scanning wheel and tilted facets thereof in the scanningarrangement of FIG. 11.

FIG. 12 is a cross-section view schematically illustrating one exampleof a projection screen for a display in accordance with the presentinvention, the screen including an array of contiguous sphericalsurfaces.

FIG. 12A is a three-dimensional view schematically illustrating furtherdetails of the screen of FIG. 12.

FIG. 13 is a three-dimensional view schematically illustrating anotherexample of a projection screen for a display in accordance with thepresent invention, the screen including two spaced-apart sheets, eachthereof including an array of cylindrical microlenses and arranged suchthat the microlenses in one array are oriented perpendicular to themicrolenses in the other array.

FIG. 14 is a three-dimensional view schematically illustrating yetanother example of a projection screen for a display in accordance withthe present invention similar to the screen of FIG. 13 but wherein thetwo arrays of cylindrical microlenses are formed on opposite sides of asingle sheet.

FIG. 15 is a cross-section view schematically illustrating yet anotherexample of a projection screen for a display in accordance with thepresent invention, the screen including an array of sphericalmicrolenses.

FIG. 16 is a three-dimensional view schematically illustrating furtherdetails of the projection screen of FIG. 15.

FIG. 17 schematically illustrates a fragment of a screen having anuneven distribution of uneven smooth surface features.

FIG. 18 is a graph schematically illustrating spatial Fourier componentamplitude as a function of feature spatial frequency for a generalizeddistribution of surface features similar to the distribution of FIG. 17.

FIG. 19 is an elevation view schematically illustrating one preferredexample of a screen including a cell containing a fluid having particlessuspended therein, and a piezo-electric transducer and one of aplurality of magnetically driven stirrers for keeping the particles inrandom motion.

FIG. 19A is a three-dimensional view schematically further detail of themagnetically driven stirrers in the screen of FIG. 19.

DETAILED DESCRIPTION OF THE INVENTION

A preferred laser for illuminating a display in accordance with thepresent invention is an optically pumped, external cavity,surface-emitting semiconductor laser, hereinafter referred to simply asan OPS-laser, includes a semiconductor chip (OPS-chip) comprising amirror-structure surmounted by a gain-structure. A mirror, separate fromthe mirror structure, forms a resonant cavity including thegain-structure. The gain-structure is usually pumped by light from adiode-laser or plurality thereof. The gain-structure includes aplurality of active layers (quantum-well layers) spaced apart bypump-light-absorbing spacer layers. The resonator may be arranged togenerate and deliver laser radiation at a fundamental wavelength, or ata harmonic wavelength of the gain-structure of the OPS-chip. Theharmonic wavelength is generated by including one or more opticallynonlinear crystals (harmonic generating crystals) in the resonator.

Examples of OPS-lasers can be found in U.S. Pat. Nos. 5,991,318;6,097,742; 6,198,756 and 6,370,168 each of which are incorporated hereinby reference.

One advantage of the OPS-laser over a solid-state laser for use in avideo display is that the wavelength of laser radiation delivered,whether fundamental or harmonic, can be selected, essentially withoutlimitation, by selecting an appropriate composition of material for theactive layers of the gain-structure. Selecting appropriate wavelengthsfor red, green, and blue radiation for a display is important inproviding faithful color reproduction.

Beam quality in a laser beam is usually characterized by a quantity M²which is a measure of beam divergence relative to a theoretical,diffraction-limited divergence (M²=1). In a practicalsingle-longitudinal-mode laser, an M² of about 1.1 is usually achieved.A problem of such a high-quality beam in a laser illuminated displayhowever is that speckle effects would be exacerbated by a high degree ofcoherence in the beam, corresponding to the high beam quality.Accordingly, for use in displays in accordance with the presentinvention, an OPS-laser is developed with a resonator configuration thatforces the resonator to deliver radiation in a plurality of transversemodes, thereby providing a lower quality, more divergent beam, that cancontribute to reducing speckle effects in the display. Beam qualitywhile relatively low by laser standards is certainly higher than thatwhich would be obtainable from conventional white light sources used inprior-art commercial projection displays. Because the laser is onlyrequire to operate in multiple-transverse-modes rather than a singlelongitudinal mode, tolerances on components can be relaxed, andresonator alignment is less critical. This significantly reduces thecost of and simplifies the manufacture of the OPS-laser. Theseadvantages are achieved in a compact efficient resonator, withoutsacrifice of output power, as disclosed in a detailed description of oneexample of such an inventive OPS-laser and the use thereof in aprojection display presented hereinbelow.

Referring now to the drawings, wherein like components are designated bylike reference numerals, FIG. 1 is a view, partially in functional blockdiagram form, schematically illustrating a first preferred embodiment 20of a laser-illuminated, rear projection television display in accordancewith the present invention. Laser 20 includes an OPS-laser 22. OPS-laser22 includes an OPS-chip (OPS-structure) 24 including a mirror-structure26 surmounted by a gain-structure 28. Gain-structure 24 is opticallypumped by pump-light (designated by solid arrowheads P) from adiode-laser array 30. Pump-light from array 30 is delivered, here, viaan optical-fiber bundle 32 and focused by a lens 34 onto gain-structure28. This method is depicted, here, for convenience of illustration.Other methods of directing pump-light to the chip may be used withoutdeparting from the spirit and scope of the present invention. Thefocused pump-light beam has a diameter on the gain-structure that isreferred to herein after as the pump-spot size or pump-spot diameter.OPS-chip 24 is bonded to a heat sink 36 that removes heat generated inthe OPS-chip by absorbed pump-light that is not converted to laserradiation.

A laser resonator 38 is formed between mirror-structure 26 of theOPS-chip and a mirror 40. Resonator 38 has a longitudinal axis(designated by dashed line 42) that is folded by a mirror 44, here, aconcave mirror. Optically pumping gain-structure 28 causes fundamentalradiation to circulate generally along resonator axis 42, as indicatedby open arrowheads F. The fundamental radiation has a wavelengthdependent on the composition of the semiconductor material from whichactive layers of gain-structure 28 are formed. Mirror-structure 26,mirror 40, and mirror 44 are highly reflective, for example, greaterthan about 99% reflective, and preferably greater than 99.9% reflectivefor the fundamental wavelength. Mirror 40 is also highly reflective fora wavelength one-half that of the fundamental-wavelength, i.e., thesecond-harmonic or 2H-wavelength. Mirror 44 is highly transparent, forexample, greater than about 95% transparent for the 2H-wavelength.

An optically nonlinear crystal 46 is located in an arm of resonator 38between fold mirror 44 and resonator end-mirror 40. Optically nonlinearcrystal 46 is arranged to frequency-double the circulating radiation andthereby generate frequency-doubled (second-harmonic or 2H) radiationhaving a wavelength one-half that of the fundamental radiation. The2H-radiation is generated on both forward and reverse passes of thefundamental radiation through the optically nonlinear crystal. The2H-beam exits the resonator via mirror 44 as a divergent beam of outputradiation. Extreme rays of the 2H-radiation beam are designated by lines50 and open arrowheads 2H. Resonator 38 is configured such that thefundamental beam size in optically nonlinear crystal 46 is focused to asmall waist to maximize the intensity of the fundamental radiation andthereby maximize, all else being equal, the 2H-conversion efficiency.

A wavelength selective element 52, here, a birefringent filter, islocated in a second arm of the folded resonator between OPS-chip 24 andfold mirror 44. This element is used to select a fundamental wavelengthfrom the gain bandwidth of gain-structure 28 of the OPS chip. An etaloncould be used in place of the birefringent filter. It is recommended,however, that the bandwidth of the wavelength selective element be onlysufficiently narrow that wavelengths outside the acceptance bandwidth ofoptically nonlinear crystal 46 be prevented from oscillating inresonator 38. If the bandwidth is made any narrower, the bandwidth2H-radiation output by the laser will be correspondingly narrowed. Thismay exacerbate speckle effects in the display. The coherence length of alaser beam is inversely proportional to the spectral bandwidth of thebeam. Accordingly, the wider the spectral bandwidth, the shorter thecoherence length of the light, and the less the possibility of speckleeffects on a projected beam of the light.

The size of the circulating fundamental beam is not indicated in FIG. 1for simplicity of illustration. Important dimensions of the fundamentalbeam are the diameter of the beam at gain-structure and the diameter ofthe beam at optically nonlinear crystal. A discussion of thesedimensions is set forth below with reference to resonator configuration,and to a desired mode mode-structure of the 2H output beam.

In prior-art OPS resonators it is customary to configure a resonatorsuch that the size of the fundamental (TEM₀₀) mode at the OPS-chip isabout the same size as, or even slightly larger than, the pump-spot.This is consistent with current practice in solid-state lasers whereinTEM₀₀ mode size and the pump-spot size are matched at the solid-stategain medium for optimum gain extraction. In an OPS laser, however,because of the periodic nature of gain distribution in thegain-structure of the chip, the first mode to oscillate commands all ofthe available gain, thereby preventing any other mode from oscillating.In a resonator that does not permit oscillation of transverse modes, theoscillating mode, accordingly, is one single longitudinal mode,spontaneously self-selected from a range of longitudinal modes ofslightly different wavelengths that could theoretically oscillate,dependent, inter alia, on the length of the resonator. It is this easeof (essentially inherent) single-mode operation, that as caused anOPS-laser to be considered by practitioners of the art as a convenientlaser for providing a high quality, single-longitudinal-mode beam. Sucha beam, however, is undesirable in a projection display, and such alaser is relatively difficult and expensive to manufacture.

In OPS laser 22, resonator 38 is configured such that the TEM₀₀ modesize at OPS-chip 24 is significantly smaller than the pump-spot size.This is accomplished in a way that allows other transverse modes to makeuse of gain available in that part of the pumped gain-structure 28surrounding the TEM₀₀ mode and oscillate in the resonator. Theresonator, accordingly, oscillates such that 2H-output beam 50 includesa plurality of transverse modes, regardless of the inherent longitudinalmode selection of the OPS-resonator. Different transverse modesoscillate and propagate at angle to each other, which, as discussed indetail further hereinbelow, can be advantageous in reducing speckleeffects.

In an OPS laser 22, the degree of transverse mode operation can bevaried by varying the spacing L between mirrors 40 and 44, while keepingthe spacing between OPS chip 24 and mirror 44 constant. A detaileddescription of theoretical and experimental results of such variation isset forth below with reference to FIG. 2, FIG. 3, and FIG. 4.

FIG. 2 is a graph schematically illustrating the variation in thecomputed spot-size of the TEM₀₀ mode at OPS chip 24, and at opticallynonlinear crystal 46 in one example of a resonator 38, as a function ofvarying distance L of mirror 40 from mirror 44. Sizes are shown for thetransverse X-axis of the resonator (curve 2B), parallel to the plane ofthe drawing, and the transverse Y-axis perpendicular thereto (curve 2A).In this resonator, mirror 40 is a plane mirror and mirror 44 is aconcave mirror having a radius of curvature (ROC) of about 100.0millimeters (mm). The distance from OPS-chip to mirror 44 is about 200.0mm. The distance L between mirrors 40 and 44 was made variable betweenabout 52.0 and 60.0 mm. Accordingly, the total optical length of theresonator (axial optical distance from mirror structure 26 to mirror 40)is between about 252.0 and 260.0 mm. The physical length of theresonator, is about 200.0 mm. The included fold angle of the resonatoris about 10°. This example of resonator 38 should operate in a singlelongitudinal (TEM₀₀) mode when L is about 57.0 mm or less.Gain-structure 28 of the OPS-structure includes active of layers ofindium gallium arsenide (InGaAs).

The gain-structure is pumped by up to about 60.0 W of diode-laser lighthaving a wavelength of about 800 nm. The diode-laser pump light in thisexperiment is supplied by two diode-laser array packages via two opticalfibers. The pump-spot size on gain-structure 28 is about 300 micronsradius. The fundamental wavelength is about 920 nanometers (nm),providing 2H-output radiation at a wavelength of about (460) nm. Thisapproximates a preferred blue wavelength in an rgb display. Heat sink 36is an air-cooled, copper heat sink, and OPS-chip 24 is soldered to theheat sink via a diamond heat-spreader layer.

The computed TEM₀₀ beam size at the OPS-chip, when L is equal to about57.0 mm is about 0.27 mm. As L increases, the TEM₀₀ beam sizeprogressively decreases until at L equal to about 66 mm, the TEM₀₀ beamsize is about 0.13 mm. The size as a function of L is not significantlydifferent in either transverse axis (see curves 2C and 2D for the Y andX-axes respectively). Over the same range of L, the beam size atoptically nonlinear crystal 46 is substantially constant, and aboutequal, in both transverse axes, at a size of about 0.05 mm.

FIG. 3 is a graph schematically illustrating measured divergence M² of2H-output beam 50 in the above described example of OPS laser 22, in thetransverse X an Y-axes of resonator 38, as a function of length of onearm of the resonator. Y-axis measurements are designated by circles, andX-axis measurements are designated by triangles. As L is increased, themeasured M² in the Y-axis increases with a progressively increasingslope. In the X-axis, the measured M² increases similarly up to a valueof about 4, at which M² remains essentially clamped. This is presumed tobe because at greater values of M², the divergence is greater than thephase-matching acceptance angle of optically nonlinear crystal 46. Atest measurement indicated that the 2H-output beam was still aboutsymmetrical in cross-section at M²=6.

FIG. 4 is a graph schematically illustrating measured total power (ofall modes) in 2H-output beam 50 as a function of M², here, Y-axis beamdivergence M² _(y), in the above-discussed example of resonator 38. AsM² increases from the single-longitudinal-mode-only condition (M²≈1.0),output power increases rapidly from an initial value of about 4.4 W,and, at an M² slightly greater than about 2.0, is about 8.25 W. i.e.,about twice the single-longitudinal-mode power. At M²=about 6, at alevel greater than which the 2H-output beam, in this example at least,becomes asymmetrical, output power is still well above thesingle-longitudinal-mode-only value. At M²=11, 2H-output power is stillhigher than the single-longitudinal-mode-only value. At this M² value,however, the 2H-output beam is likely to be highly asymmetrical. Beamasymmetry in this application is not a particular disadvantage and couldbe an advantage in illuminating a one-dimensional spatial lightmodulator. Accordingly, a useful M² range for the inventive OPS laser isbetween about 2 and 20.

The OPS-laser used for the above-described experiments to determine auseful range of M² for a multiple-transverse-mode laser in accordancewith the present invention has a disadvantage that the overall opticallength of the resonator at about 260.00 mm and indeed the physical“footprint” of the resonator at about 200.0 mm is longer than would beconvenient in a practical commercial projection display. Accordingly, adifferent resonator was designed, consistent with the arrangement ofresonator 38, but that is more compact and that will only operate inmultiple transverse modes, and more specifically could not be rearrangedto operate in a single mode.

In this example of resonator 38, mirror 44 has an ROC of about 75.0 mmand mirror 40 is a plane mirror. Distance L between mirrors 44 and 40 isabout 43.0 mm. The distance between OPS chip 24 and mirror 44 is about60.0 mm. The resonator fold angle is about 30°. The resonator deliverspeak CW 2H-power of about 6.0 W for a pump power of about 50.0 Watts, atan M² between about 2 and 3. Here, the resonator output power iscomparable, at comparable M², to the first-discussed resonator exampleused in the above described experiments, while the physical length ofthe resonator, at about 60.0 mm, is less than one-third the length ofthat first discussed resonator example.

It should be noted that in this compact version of themultiple-transverse mode OPS resonator, pump-light is provided by adiode-laser bar (linear array of diode-lasers) and focused directly fromthe diode laser bar onto OPS chip 24. The fast axis of the diode-laserbar is aligned in the X axis of the resonator. Focusing is accomplishedby a total of four lens elements, with two of the elements being used tocollimate light from the diode laser bar, and the other two elementsbeing used to focus the collimated light. The focused spot is somewhatrectangular in shape. This provides an increase in pump-light to outputefficiency (compared with fiber delivered pumping) ranging from about100% at a relatively low output power of about 1.5 W to about 30% at anoutput power of about 4.5 W.

It is believed that, for this particular resonator configuration atleast, even greater efficiencies can be achieved if the light from thediode-laser bar is focused to a pump spot having an elliptical shape,and a power density of about twenty kilowatts per square centimeter (20KW/cm²) on the OPS-chip. This may require the use of aspheric lenses.However, the manufacturing and design technology for such lenses is wellwithin the capabilities of commercial concerns that specialize inoptical design and manufacture.

It should be noted that a further advantage of such pumping directlyfrom a diode-laser bar is that considerably less space is required forpump-light-delivering apparatus than would be required for one or morecommercial diode-laser array packages from which light is delivered viaan optical fiber bundle. This space-saving aspect of direct pumping isconsistent with the quest for compactness without sacrifice of power ina laser for projection display applications.

It should further be noted that while the multiple-transverse-modelasers in accordance with the present invention are described above interms of a frequency-doubled laser, such laser being designed for andused in a projection display, red, green or blue light may also beprovided by the fundamental wavelength of a multiple-transverse-modeOPS-laser. Green light may be generated from an OPS-laser chip includingactive layers of a II-VI semiconductor, such a zinc sulfo selenide(ZnSSe). Blue light may be generated from an OPS-laser chip includingactive layers of an indium gallium nitride (InGaN). At the present stateof development of these materials however, it may not be possible togenerate the desired one Watt or more of power from a diode-pumpedOPS-laser using these materials.

Continuing now with reference again to FIG. 1, laser 22 of display 20includes two other lasers (not explicitly) shown, that provide the tworgb colors not provided by laser 22. These two lasers are preferablymultiple transverse OPS-lasers as described above. Output beams fromlaser 22 and from lasers 2 and 3 are combined in an optical colorcombining arrangement, here, a well-known Philips prism arrangement 54.Beam contributions from lasers 2 and 3 are designated by double andtriple open arrowheads, respectively.

The combined laser beams are focused by a lens 58 into a beamhomogenizer 60.

Homogenizer 60 may be any type of diffuser including a light pipe, anoptical fiber and a diffuser. A light pipe, however, is preferred foruse with the inventive multiple-transverse-mode OPS-laser. Whilehomogenizing combined beams in a single homogenizer is preferred, eachlaser beam may be individually homogenized without departing from thespirit and scope of the present invention.

In addition to the inventive multiple-transverse-mode OPS-laser 22,display 20 includes usual arrangements 62 for converting light fromthree lasers into a projected image of a display. The arrangementsinclude video electronics for receiving broadcast or media-recordedvideo image information; a spatial modulation (spatial amplitudemodulation) arrangement for applying light-intensity values to theilluminating laser beams according to the received video information,and an arrangement for time modulation of the laser beams for sequencingbeams of the different primary colors into the spatial modulationarrangement. These arrangements are designated in FIG. 1 as beingincluded in a single unit 64 for convenience of illustration. Thearrangements, however, may be in separate units, and the functionsprovided need not be located in, or performed in, the sequencedesignated in FIG. 1.

By way of example, in one conventional display designed for (non-laser)white light illumination, a so called color-wheel including a peripheralarray of color filters is arranged to sequentially transmit the threeadditive primary colors (rgb) and interposed between the spatial lightmodulator and the illumination source. This functions as a chopper-wheeltype of time modulator that, in effect, sequentially turns theilluminating beam (from the point of view of the spatial modulator) onand off, with the color changing to one of the primary colors at each“on” period. Each primary color is modulated separately by the spatialmodulator to correspond to the spatial content of that color in theprojected image on screen 68. The modulation rate is sufficiently rapidthat, to an observer, all three colors appear to be presentsimultaneously in the display. In the laser of FIG. 1, where individualCW beams of red green and blue light are provided, these beams may besequenced on and off individually by some other time modulationarrangement such as a simple chopper wheel, an acousto-optic (AO)modulator, or an electro-optic (E-O) modulator.

It is emphasized, here, that image providing arrangements depicted inFIG. 1 are not intended to represent any particular one sucharrangement. The arrangements will depend in particular on the type ofspatial modulator or modulators employed, and whether these are one ortwo-dimensional modulators. One-dimensional spatial modulators (spatiallight modulators) will require a scanning arrangement to provide asecond dimension to the projected image. Spatial modulators may include,among others, a digital light processor such as a DLP™ produced by TexasInstruments of Dallas Tex., a grating light valve such as a GLV™produced by Silicon Light Machines of Sunnyvale, Calif., a gratingelectromechanical system such as a GEMS™ produced by Eastman Kodak ofRochester, N.Y., or a liquid crystal based modulator such as LCOS™(liquid crystal on silicon) produced by Intel Corporation of SantaClara, Calif. As the operation of these modulators and arrangements forprojecting an image using each of these modulators has been described indetail in prior-art documents, a detailed description of any suchoperation or arrangement is not provided here.

FIG. 5 schematically illustrates a second preferred embodiment 70 of alaser-illuminated, rear projection television display in accordance withthe present invention. Display 70 is similar to display 20 of FIG. 1,with an exception that time modulation of light illuminating the displayis effected by directly modulating the operation ofmultiple-transverse-mode laser 22 rather than modulating CW radiationdelivered by the laser after the radiation has been delivered. Indisplay 70, multiple-transverse-mode laser 22 is pumped by a modulateddiode-laser array 33. A video unit 63 provides signals controllingmodulation of the diode-laser array, the term “modulation” in thisinstance meaning turning the diode-laser array on and off, i.e.,alternately delivering and not delivering pump-light. Laser 22,correspondingly, intermittently delivers radiation, in effect, as asequence of radiation pulses, corresponding to the modulation ofdiode-laser array 33.

In time modulation of a CW (unmodulated) beam delivered by a laser,radiation in the “off” periods for the radiation is directed away fromthe spatial modulators and projection optics of the display and must beabsorbed or baffled in such a way that the quality of the projecteddisplay is not adversely affected. In the above described directmodulation scheme, laser 22 only delivers radiation that will bespatially modulated for projection. This is advantageous in minimizingthe need for absorbing or baffling “unprojected” radiation and is alsoadvantageous in saving power consumed in generating radiation that doesnot form part of a displayed image. Further, when an OPS-structure isdirectly modulated, pump-light is only delivered to the OPS-structurewhen radiation is required. This reduces heat deposited in the structurecompared to CW delivery of pump-light and external modulation. Thisadvantage can be exploited to provide higher pulse power on a given heatsink, or to reduce heat sink requirements for the same pulse power.

By way of example FIG. 5A schematically illustrates peak 2H-output poweras a function of peak pump-light power in multiple-transverse modeOPS-laser in accordance with the present invention, for CW operation(dashed curve), and for pulsed (modulated) operation (solid curve). Inthe pulsed operation, radiation is delivered for one millisecond (ms)with a two milliseconds “off” time between deliveries, i.e., in a dutycycle of 1:3. It can be seen that in CW operation, output powerincreases with increasing pump power before falling catastrophically asthe OPS-chip reaches the exceeds a maximum-possible operatingtemperature. In pulsed (directly modulated) operation, power increasesessentially linearly with pump power, at least for the range of poweravailable for the experiment.

CW performance could possibly be improved by improved-heat sinking,however, any such improvement would also be obtained in directlymodulated operation. A detailed description of heat sinking techniquesis not necessary for understanding principles of the present inventionand is not presented herein. In experiments described above, OPS-chipswere bonded to commercially-available heat sinks. These are believed tobe constantly under development by manufacturers of same, and multipletransverse mode OPS-lasers in accordance with the present may benefit,one way or another, from any improvements that result.

Continuing with a discussion of advantages of direct modulation of amultiple-transverse-mode OPS laser, in a display including aone-dimensional spatial modulator and scanning projection optics,modulation rates may be as high one megahertz (MHz), dependent, interalia, on the number of scan lines in a frame of the image and the framerefresh rate. A determining factor in how rapidly a laser can bedirectly modulated by modulating pump-light delivered to the gain-mediumof the laser is a so called “relaxation time” characteristic of thegain-medium. The relaxation time is the time required for gain generatedby a pump-light pulse to decay after the pulse is terminated. In an OPSgain-structure, the relaxation time is less than 1.0 microsecond (μs)and can be as little as one-hundred nanoseconds (ns).

A diode-laser array for providing pump-light can be modulated at ratesas high as a few MHz, this allows the inventive directly modulatedOPS-laser to be directly modulated at rate of at least 1 MHz andpossibly as high as a few MHz. In a solid-state gain-medium such asneodymium-doped YAG (Nd:YAG) or neodymium-doped yttrium vanadate(Nd:YVO₄) the relaxation time is on the order of one-hundredmicroseconds (μs). This limits direct modulation of such a gain-mediumto a maximum of about 100 kilohertz (KHz), whether or not thegain-medium is pumped by a diode-laser array.

In an experiment to test the efficacy of the inventivemultiple-transverse-mode OPS-laser in reducing speckle effects in alaser-illuminated projection video display, an experiment was conductedin which green and blue multiple-transverse-mode OPS-lasers configuredin accordance with the configuration of laser 22 of FIG. 1, and adiode-laser bar providing red light were used to illuminate acommercially available projection display that was originally designedto be illuminated by incoherent “white” light from a mercury lamp.

The lasers used in the experiment were CW lasers. i.e., not directlymodulated. The original mercury-lamp illuminated display wasmanufactured by Samsung Corporation. Such a display, being configuredfor illumination by incoherent light, does not include any device ormeasures expressly for reducing speckle effects of coherent light. Themercury lamp of the laser was removed and the combined output of themultiple-transverse-mode OPS-lasers substituted as a source of whitelight for illuminating the display. Output beams from the lasers arecombined by a dichroic combiner and focused into a beam homogenizer asdepicted in display 20 of FIG. 1. The homogenized beam was relayed intoanother beam homogenizer (not shown) already provided in the display.

While speckle contrast viewable in a projected image could be judged aspossibly somewhat less than what might be expected in a displayilluminated by single-longitudinal mode lasers of limited spectralbandwidth, speckle effects were still readily perceivable. Theterminology “readily perceivable” as used here means that an observerinvited to view the display, without specifically being directed toexamine the display for speckle effects, would be conscious of theseeffects and may be distracted by the effects unless the subject matterof the display were sufficient to completely capture the observer'sinterest.

One effect that was noticeable, in addition to speckle contrast, wasthat, in a blank-white screen-image, there are readily perceivablecolored “rainbow” effects. It is believed that these effects result fromdiffraction of the illuminating light at any particle or scratch in oron an optical component in the chain of projection optics. The effectappears as a series of rainbow colored lines and circles. Specklepatterns are different for each projected color and produce a finelydispersed rainbow effect all over an otherwise blank white display.

The rainbow effects were eliminated in a modification of theexperimental display schematically depicted in FIG. 6 and FIG. 6A. Herean apparatus 72 includes elements of the above-discussed commercialprojection TV (less the originally-provided mercury lamp) grouped asdesignated by dotted line 73.

The red, green, and blue lasers used for illuminating the TV are notexplicitly depicted in FIG. 6 but are designated simply as lasers 1, 2,and 3. Beams from these lasers are combined in Philips prism 54 toprovide a combined beam 56 having a divergence Θ_(L). Beam 56 isdirected to a two-dimensional scanner arrangement 74 comprising firstand second faceted scanner wheels 76 and 78, rotatable about mutuallyperpendicular axes as indicated by arrows A and B. The scanner wheelsare rotated by precision stepper motors 79 and 81 (FIG. 6A). Whenscanner wheels 76 and 78 are rotated, beams comprising beam 56 arescanned through a range of angles in planes parallel and perpendicularto the plane of the drawing. The beams are scanned over a condenser lens71 including plano-convex lens elements 75 and 77. Condenser lens 71focuses beam-scans in a plane perpendicular to the plane of FIG. 6 backto a single spot on a facet scanner wheel 78 (see FIG. 6A). Afterreflection from that facet, of course, the beam scans will again fan outin the plane perpendicular to the drawing of FIG. 6 while being scannedin the plane of that drawing. Extreme positions of the scanned beam-fansin the plane of FIG. 6 are indicated by dotted lines 56A and 56B.Extreme positions of the scanned beam in the plane of FIG. 6A(perpendicular to the plane of FIG. 6) are indicated by dotted lines 56Cand 56D.

Scanning of beam 56 by the scanner wheels was arranged such that thebeam was scanned in a raster fashion over lens 80, with scanning by onefacet of scanner wheel 78 being completed at about the frame rate of thedisplay 73, while scanning by one facet of scanner wheel 76 wascompleted at about the line rate of the display. Use of faceted scanningwheels permits a high scanning rate, with a required rotation rate ofeach scanner wheel being a fraction of the scanning (line or frame) rateinversely dependent on the number of facets on that wheel. Thispermitted that a motor driving a scanner wheel, for example, motor 79driving scanner wheel 76, as noted above, could be a precision steppermotor. The rotation rate of such motors can be accurately controlled.Beam 56 was scanned through an entire raster pattern at the frame rateof the display, i.e., in less than about 50.0 ms, which is less than theresponse time of the human eye.

The scanned combined laser beams were focused by a condenser lens 80(including plano-convex lens elements 82 and 84) into homogenizer 60 ofdisplay 73. The effect of the scanning is that the combined beams,averaged over time, appear to an observer to fill a cone or solid anglebounded by an angle Θ_(A), which is the acceptance angle of the display,and is greater than the laser beam divergence Θ_(L). A result of thescanning was that speckle effects, while not entirely eliminated, wereno longer readily perceivable by a casual observer. The rainbow effectsobserved without the scanning arrangement were completely eliminated.

Continuing with reference to FIG. 6, it is useful at this point tobriefly review the principle of increasing beam divergence to reducecoherence related effects of a beam such as the above-discussed speckle(interference) and rainbow (diffraction) effects. The maximum angle ofthe light cone Θ_(A) of condenser lens is limited by the acceptanceangle of the particular spatial light modulator used in the display. Inaddition, dimensions of the light modulator and of the screen determinethe maximum angle Θ_(S) subtended by the projection optics on screen 86.Angle Θ_(S), accordingly, is proportional to angle Θ_(A) and determinesthe spatial coherence properties of light incident on the screen.Specifically, the coherence radius r_(c) at the screen is approximatelyequal to λ/Θ_(S), where λ is the wavelength of the monochromatic light.By way of example, in the display used in the experiment, the spatiallight modulator is a DLP™ modulator having a diagonal of about 0.8inches. Screen 68 has a diagonal of about 52 inches. Optics illuminatingthe modulator can have an f:number of about 2.4. This means that Θ_(S)is about equal to 2*ArcTan(1/(2*f:number), which is about equal to 23.5degrees. The angle Θ_(S) subtended by the projection optics at thescreen is about equal to Θ_(A)*0.8/52, which is equal to 0.36 degrees.This leads to a coherence radius r_(c) of about 80.0 μm for a wavelengthλ of 0.5 μm.

The contrast of speckle observed by a standard observer is proportionalto the ratio of the coherence radius r_(c) to the size of the pointspread function of the observer's eye at the screen. One way to reducespeckle contrast is to reduce the spatial coherence radius r_(c) of theincident light by actually increasing, or effectively increasing, itsangular spread, ultimately, to match the acceptance angle of theprojection optics.

Lasers generate highly spatially coherent beams. High spatial coherence,here, means that the radius of spatial coherence is on the order of thebeam diameter. The coherence radius is in inverse proportion to thedivergence angle. The divergence angle of a single-mode laser beam isnearly diffraction limited. The diffraction limited divergence angle isfar below the acceptance angle Θ_(A) of a typical projection TV asdiscussed above. Increasing the apparent divergence angle of a laserbeam substantially above the original divergence angle thereof by theabove-discussed scanning arrangement reduces coherence radius of thebeam at the screen compared with the original coherence radius of thebeam, thereby contributing to reducing speckle contrast in a projectedimage.

In order to appreciate the contribution of the multiple-transverse-modeOPS laser to reducing speckle contrast it is useful to consider thefollowing simple empirical explanation of how speckle contrast isreduced by divergent beams. If it were possible to observe monochromaticlight incident on the screen in any one direction there would beobserved a high speckle contrast. When light arrives on the screen in arange of directions there can be high speckle contrast in any onedirection, but, as interference patterns causing the speckle effect arenot in phase, these patterns average each other out to a point where, ifthere are enough patterns involved in the averaging, there will be noperceived speckle effects.

Now considering the divergence of the laser beam, any one bundle of raysincident on condenser lens 80 will not be a collimated bundle of raysbut will be divergent to an extent dependent, inter alia, on the numberof trasverse modes in which the multiple-transverse-mode laser isoperating. The additional divergence of the laser beam adds to thenumber of directions from which light is incident on the screen andaugments the speckle-contrast reduction provided by the scanningarrangement. Clearly, of course, as the laser beam divergence isincreased, there can be reached a divergence at which power in the beamfalls to inadequate levels, or the beam becomes of sufficiently poorquality (sufficiently high M²) that it can not be adequately focused bythe projection optics. As discussed above, however, with reference toFIG. 3 and FIG. 4, a multiple-transverse-mode OPS-laser in accordancewith the present invention can be operated at an M² of as high as about6.0 without undue asymmetry or reduction of power and at even higher M²if asymmetry is not a problem. At an M² of 6.0, the beam would stillhave more than sufficient quality to be adequately focused, consistentwith even the demands of high definition television (HDTV).

It should be noted here that scanning arrangement 74 of FIG. 6 is notthe only scanning arrangement that can be used to accomplish apparentincreased beam divergence in a display in accordance with the presentinvention. FIG. 7 schematically illustrates a considerably simplerarrangement 86, including a wedge or prism-shaped element 88 that isrotatable about an axis (not shown) perpendicular to one face thereof,as indicated by arrow C. In this drawing and in other drawingsdescribing other scanning arrangements described hereinbelow only thescanning arrangement and the condenser lens receiving the scanned beamare shown for economy of illustration. Rotation of element 88 causescombined laser beams 56 to be scanned into an annular path on lens 80,into a hollow cone in effect, as indicated by dotted beams 56A and 56B.This arrangement requires that prism element is rotated at a much higherrate than faceted wheels 76 and 78 of scanning arrangement 74 of FIG. 6.Rotation of the element is further complicated inasmuch as it must beeffected by an edge driven holder (not shown) of the element. Further,as the scanned beam describes only an annulus at the condenser lens anddoes not fill the cone, the angle of which is bounded by beams 56A and56B. This results in a lesser reduction of the coherence radius thanwould be the case if the scanned beam, averaged over time, filled theentire cone as is the case in scanning arrangement 74.

One shortcoming of scanning arrangement 86 is remedied in anotherscanning arrangement 90 depicted in FIG. 8. Here, combined laser beams56 are incident on a stationary tilted plane mirror 92. Mirror 92directs the combined beams onto another plane mirror 94 that is mountedon a drive shaft 96 of a drive motor 98, with the plane of the mirror atan angle to a perpendicular to the drive shaft. Rotation of mirror 94 asindicated by arrow D causes combined laser beams 56 incident thereon tobe scanned into an annular path or cone as indicated by dotted beams 56Aand 56B. This arrangement does not remedy the absence of cone-filling orthe rotation speed shortcomings of scanning arrangement 74.

FIG. 9, schematically illustrates yet another scanning arrangement 100.Scanning arrangement 100 is similar to scanning arrangement 90 with anexception that stationary mirror 92 of that scanning arrangement isreplaced by a mirror 102 mounted on a drive shaft 104 of a drive motor106, with the plane of mirror 102 at an angle to a perpendicular to thedrive shaft. A condenser lens 71, including plano-convex lens elements 5and 77, is located between the rotating mirrors and serves to refocusbeam scans from mirror 102 back to a single spot on mirror 94. Rotatingmirror 102 as indicated by arrow E, while rotating mirror 94 asindicated by arrow D, scans an annular pattern created by the mirror 102into another annular pattern. If the mirrors are rotated at differentrates this causes beams in combine beam 56 to be scanned in overlappingspiral patterns on condenser lens 80. This effectively, averaged overtime, fills the cone bounded by the scan angle.

FIG. 10 schematically illustrates still another alternative scanningarrangement 110 for simulating increased angular divergence of combinedlaser beams 56. Scanning arrangement 110 includes galvanometer scanmirrors 112 and 114. Scan mirror 112 is mounted on a drive shaft 116 ofa motor a motor 118. Mirror 112 is scanned in reciprocal rotation aboutan axis (not explicitly shown) in the plane of the drawing as indicatedby double arrows F. Scan mirror 114 is mounted on a drive shaft 120 of amotor a motor 122. Mirror 114 is scanned in reciprocal rotation about anaxis (not explicitly shown) perpendicular to the plane of the drawing,i.e., perpendicular to the scanning axis of mirror 112, as indicated bydouble arrows G. A condenser lens 71, including plano-convex lenselements 75 and 77, is located between the galvanometer scan mirrors andserves to refocus beam scans from mirror 112 back to a single spot onmirror 114. This arrangement allows a raster-type scan similar to theraster scan of scanning arrangement 74 of FIG. 6. Preferably, mirror 114is scanned at the frame rate of the display, while mirror 112 is scannedat the line rate of the display.

FIG. 11 and FIG. 11A schematically illustrate a further alternativescanning arrangement 130 for simulating increased angular divergence ofcombined laser beams 56. This arrangement is similar to arrangement 74of FIGS. 6 and 6A, with exceptions that faceted scanning wheel 76 isreplaced by a plane mirror 92; condenser 71 of arrangement 74 isomitted; and scanning wheel 87 of arrangement 74 is replaced inarrangement 130 by a different faceted scanning wheel 132.

Scanning wheel 132, in this example, has 20 facets in total. Thisprovides that there is an angle of 18° between each facet in the planeof FIG. 11. Facets are arranged in four sequential groups of fivedesignated 134A, 134B, 134C, 134D, and 134E in peripheral sequence.Within group 134A-E, facet 134C is parallel to rotation axis 138 ofwheel 132, the rotation axis here being perpendicular to the plane ofFIG. 11. Facets 134B and 134D are tilted by +4° and −4.5° respectivelyto rotation axis 138. Facets 134A and 134E are tilted by +9° and −9°respectively to rotation axis 138. With the facets thus configured, themaximum angle between the facets within the group with respect to therotation axis is 18°. This is equal to the angle between adjacent facetsin the rotation plane. Because of this, a beam 56, after reflection fromwheel 132 during a revolution thereof, forms an approximately squarepattern or raster consisting of five lines with angular span of +/−18degrees both parallel to the plane of FIG. 11 (beams 56A and 56B) andperpendicular to the plane of FIG. 11 (as depicted in FIG. 11A by beams56C and 56D). In terms of the number of components, scanning arrangement130 is much simpler than the two-wheel arrangement of FIGS. 6 and 6A,and synchronization of the scan-raster with the display-raster issimpler as only one motor rotation need be precisely controlled. Wheel132, however, is somewhat more difficult to manufacture than a facetedscanning wheel in which all facets are parallel to the rotation axis ofthe wheel.

It should be noted that the above-discussed scanning arrangements arenot meant to constitute an exhaustive collection of such arrangements.Those skilled in the art, from the descriptions presented above maydevise other such scanning arrangements without departing from thespirit and scope of the present invention.

While the combination of a multiple-transverse-mode OPS-laser with ascanning arrangement to create an apparent increase in divergence angleof a beam therefrom is effective in eliminating readily perceivablespeckle effects, and also diffraction related effects, in the inventivelaser illuminated display, it is believed that further reduction inspeckle contrast may be necessary to bring the speckle contrast to asufficiently low level to satisfy knowledgeable practitioners of thehigh-quality video display art. In order to achieve such a furtherreduction effort has been applied to devising screen configurations thatcan contribute to reduction of speckle effects.

Two configurations are described below. One of the configurations is ascreen comprising at least one plurality of microlenses or raisedsurface features in general. The other configuration is a screen in theform of a flat cell containing particles in suspension that areconstantly agitated by a radio-frequency (RF) transducer or the likewhen an image is being projected on the screen. It is emphasized here,however, that each of these configurations is intended to be used incombination with either or both of the above-described inventivemultiple-transverse-mode OPS-laser, or an above-described scanningarrangement, each of which provide a reduced coherence radius r_(c) atthe screen. Any of the below-described screens can be substituted forscreen 68 in above-described embodiments of the inventive display

FIG. 12 is a cross-section view (with traditional cross-hatching omittedfor clarity) schematically illustrating important dimensions of ahypothetical screen fragment 140 comprising a plurality of features (forexample spherical microlenses) 142 each having a convex surface 144.Here, features are arranged in a regular array thereof with surfacespoint-contiguous (see FIG. 12A). Coherence radius reduction resultingfrom real or apparent increases in divergence of light illuminating theinventive display provides that the spatial coherence radius r_(c) ofthe light incident on the screen is small compared to the spot size atthe screen resolvable by a viewer. This spot size is usually referred toas the point spread function (PSF) of the human eye and is dependent onthe eye-pupil (hereinafter simply pupil) and the distance of the viewerfrom the screen. By way of example, coherence radius at the screen canbe reduced to 40 μm as discussed above. The PSF radius r_(PSF) for lightof a wavelength λ, for a viewer with a pupil size D of about 3.0 mm, ata distance of L=3 meters, can be defined by an equation:r_(PSF)=λ*L/D=0.5 mm.  (1)

In screen 140 convex features 142 have a controlled convex shape. Eachconvex surface or lens 144 has a width dimension d that satisfies theinequality:r_(c)<d<r_(PSF).  (2)

i.e., the size of the lens is greater than the coherence radius but lessthan the radius of the point spread function of the viewer's eye. Eachlens has a focal length f. Light rays from the projection opticsreaching screen 40 in any one direction are redirected by one or morefeatures 142 in a plurality of directions, as indicated in FIG. 12 bysolid and dashed lines, to one or more viewers. The redirected raysoriginating from a particular area of the screen of the size of thesmaller of r_(c) or r_(PSF) add together on the amplitude basis in animage plane, i.e., a viewer's retina. Were the screen a rough screen,phases of these rays would vary unpredictably, thus creating randomintensity variations from spot to spot, i.e., speckle. However, ininventive screen 140 the rays that reach a viewer are formed by thewell-defined optical surfaces 144, and the smallest distance separatingany pair of rays propagating from an area of size r_(PSF) at the screento the viewer, is equal to the period d of the array of features(lenses) 142. Because of this, if d is greater than the coherence radiusr_(c), all of these rays are un-correlated, and accordingly are added atthe image plane on an intensity basis, in which case no speckle occurs.Since the size d of features 142 is less than the resolvable spot sizer_(PSF), a viewer cannot detect the presence of discrete features.Additionally, averaging of multiple rays originating from the resolvablespot of the size r_(PSF) helps to improve perceived uniformity of thescreen.

Preferably, the ratio of the focal length f of the features (lenses) inthe array to the lens size d is kept relatively low, in order toincrease viewing angle of the screen. For example, ratios of about 0.5are practically achievable. This corresponds to a full viewing angle of90°. While a lesser viewing angle may be considered less desirable fromthe point of view of accommodating several viewers, an advantage of areduced viewing angle is an increased gain of the screen, which allows ahigher perceived image brightness.

FIG. 13 schematically illustrates one practical arrangement 148 forrealizing a screen as described above with reference to FIG. 12. Screen148 comprises two regular arrays 150 and 152 of cylindrical microlenses.Only small fragments of what would be the practical extent of sucharrays are depicted for convenience of illustration. Cylindricalmicrolenses 154 in array 148 are contiguous and have a spacing d asspecified above. Cylindrical microlenses 156 in array 152 are contiguousand also have a spacing d, as specified above. Arrays 150 and 152 arearranged with cylindrical microlenses in one thereof perpendicular tothe cylindrical microlenses in the other. This provides an opticaleffect similar to that which would be provided by a regular array ofspherical microlenses having a diameter d and focal length f, and beingcontiguously packed in the array. Referring to FIG. 14, a similaroptical effect can be obtained in a screen 158 wherein theperpendicularly oriented arrays of cylindrical microlenses 154 and 156are impressed on opposite sides of a single sheet 160.

FIG. 15 is a cross-section view (again with traditional cross-hatchingomitted for clarity) schematically illustrating important dimensions ofa hypothetical screen fragment 162 comprising an array of convexspherical microlenses 164, spaced apart by a distance d and having afocal length f, as specified above. FIG. 16 is a three-dimensional viewschematically illustrating screen fragment 162 including atwo-dimensional array of the convex spherical microlenses.

Arrays of spherical and cylindrical microlenses having featuredimensions described above can be made by molding or printing the arraysinto transparent plastic sheets. Similar arrays are used in industry asdiffractive optical elements for information technology, industryautomation and biomedical applications. Such arrays are commerciallyavailable, for example, from Edmund Industrial Optics of Barrington,N.J., and Leister Technologies LLC of Itasca, Ill.

It is not necessary that features of a screen in accordance with thepresent invention be regular as discussed above. By way of example, FIG.17 schematically depicts a surface fragment of a screen 170 havingunevenly distributed smooth features 172 of uneven circumferential shapeand height. Such a surface can be computer-generated, in a mannersimilar to that in which prior-art holographic diffusers for shapinglaser-beams into various forms are generated. Such a diffuser can bedesigned to produce a nearly uniform angular distribution of light froma beam incident thereon, thereby providing a large viewing angle for thescreen. Any algorithm used to computer generate a suitable distributionof surface features for that purpose must be constrained to restrict thespectrum of spatial frequencies ν of the feature shape or phase profile.Specifically, it is necessary that the amplitudes of Fourier componentsA(ν) of the spatial frequency spectrum are non-zero only in a spatialfrequency range below r_(c) ⁻¹, where r_(c) ⁻¹ is the inverse of thecoherence radius r_(c). This condition can be mathematically specifiedby the following equations:|A(ν)|≧0 if ν<(r_(c))⁻¹  (3)|A(ν)=0 if ν≧(r_(c))⁻¹.  (4)and is schematically graphically depicted shown in FIG. 18, whereincurve 174 can have any shape required to provide a desired angulardistribution, provided equations (3) and (4) are satisfied. Theseequations mean in effect that the smallest feature size of the screen isgreater than the coherence radius r_(c), and the discussion presentedabove for regular microlens arrays is applicable i.e., the inequality ofequation (2) must be satisfied. A description of design algorithms forsuch surfaces is not necessary for understanding principles of thepresent invention and, accordingly, is not presented herein. Surfaces ofthis kind can commercially designed and generated on a variety oftransparent materials including plastics and fused silica, for exampleby MEMS Optical Inc. of Huntsville, Ala.

Continuing now with a description of an alternate screen configuration,FIG. 19 and FIG. 19A schematically illustrate one embodiment of a screen180 in the form of a flat cell 182 containing atrasparent liquid (fluid)192 having particles 193 in suspension therein. Cell 182 has front andrear (from a viewer's perspective) trasparent panels 184 and 186, a basecap strip 188, a top cap strip 190, and side panels 185 and 187 (seeFIG. 19A). A plurality of stiring blades 194 disposed long the peripheryof the display agitate the suspended particles when an image is beingprojected on he screen. This maintains the particles in rapid randommotion with respect to each other. Each stirring blades 194 is driven bymagnetic drive unit 196 (see FIG. 19). Such stirrers are commerciallyavailable, for example from Edmund Scientific Inc. of Barrington, N.J.Such stirrers may be driven at rotation rates up to 2500 revolutions perminute. The random motion (or vibration) of the particles reducesspeckle effects. At any infinitesimal instant in time there would be aspeckle effect, for reasons discussed above with reference to a speckleproduced by a rough surface. Because of the random motion of particles,however, many speckle patterns are time averaged due to the slowresponse time of the human eye. This reduces the speckle contrast.

The particles can be, for example, particles of any commonly availableoxide material, such as silicon dioxide, aluminum oxide, or titaniumoxide. The fluid can be water, or some organic fluid of relatively lowviscosity. Whatever fluid is selected it should preferably be chemicallyinert, non-toxic, and should also preferably have a low evaporation rateand low cost.

The particles must be small enough to scatter light, for example,preferably have a dimension less than about 10.0 μm at which dimensionthe particles should not readily settle. If the particles aresufficiently small, for example, have a dimension less than about 1 μm,the particles may move randomly with respect to each other throughBrownian motion. In order to discourage particles from settling when notbeing agitated, it may be found convenient to provide one or more RFdriven transducers 198. Such transducers can be operated to preventparticles from coagulating or settling, whatever the particle size.

It should be noted here that while a random-particle-motion screen iseffective in itself, it may be used together with microlens effects asdescribed above. By way of example microlenses or similar features maybe formed on one or both of front and rear panels 186.

The present invention is described above in terms of a preferred andother embodiments. The invention is not limited, however, to theembodiments described and depicted. Rather, the invention is limitedonly by the claims appended hereto.

1. A projection display, comprising: at least one laser for delivering alight beam, said light beam of having an original beam divergence and anoriginal coherence radius, and propagating along a beam path; a beamhomogenizer; a first lens; a scanning arrangement for scanning saidlight in beam in a particular pattern over said first lens, such that,averaged over a time less than about the response time of a human eye,said light beam has an effective beam divergence greater than saidoriginal divergence thereof, said scanned beam being delivered from saidfirst lens into said beam homogenizer to be homogenized thereby; aspatial light modulator, said spatial light modulator arranged toreceive said scanned light beam homogenized by said beam homogenizer andspatially modulate said homogenized scanned light beam in accordancewith a component of an image to be displayed; a screen for displayingsaid image; projection optics for projecting said homogenized scannedlight beam onto said screen, said homogenized scanned light beam at saidscreen having a reduced coherence radius less than said originalcoherence radius thereof as a result of said scanning thereof, saidreduced coherence radius providing a first contribution to minimizingspeckle contrast in said image displayed on said screen; and wherein,said screen includes one or more features for providing a secondcontribution to minimizing speckle contrast in said image displayedthereon.
 2. The display of claim 1, wherein said scanning arrangementincludes a rotatable prism disposed in said beam path in a manner suchthat when said prism is rotated said light beam is scanned in an annularpattern over said first lens.
 3. The display of claim 1, wherein saidscanning arrangement includes a fixed mirror disposed in said beam path,and directing said common path onto a rotatable mirror arranged at anangle to a rotation axis thereof in a manner such that when saidrotatable mirror is rotated said light beam is scanned in an annularpattern over said first lens.
 4. The display of claim 1, wherein saidscanning arrangement includes first and second rotatable mirrors, eachthereof inclined to a rotation axis thereof and having a second lenstherebetween, said first rotatable mirror being disposed on said beampath in a manner such that when said first mirror is rotated said lightbeam is scanned over said second lens in a first annular pattern, saidsecond lens being arranged to focus said first annular pattern onto saidsecond rotatable mirror, and said second rotatable mirror being arrangedsuch that rotation thereof, while said first mirror is rotating causessaid first annular pattern to be scanned over said first lens in asecond annular pattern.
 5. The display of claim 1, wherein said scanningarrangement includes first and second rotatable faceted scanning wheelshaving a second lens therebetween, said facets of said first rotatablescanning wheel being disposed on said beampath in a manner such thatwhen said first scanning wheel is rotated said light beam is scannedover said second lens in a first transverse axis, said second lens beingarranged to focus said scanned beam onto facets of said second rotatablescanning wheel, and said second scanning wheel being arranged such thatrotation thereof, while said first scanning wheel is rotating, causessaid focused scanned beam be scanned over said first lens in a secondaxis perpendicular to said first axis, whereby said light beam isscanned in a raster pattern over said first lens.
 6. The display ofclaim 1, wherein said scanning arrangement includes first and secondreciprocally rotatable galvanometer mirrors having a second lenstherebetween, said first galvanometer mirror being disposed on said beampath in a manner such that when said first galvanometer mirror isrotated said light beam is scanned over said second lens in a firstaxis, said second lens being arranged to focus said scanned beam ontosaid second galvanometer mirror, and said second galvanometer mirrorbeing arranged such that rotation thereof causes said focused scannedbeam to be scanned over said first lens in a second axis perpendicularto said first axis, whereby said red, green blue beams are scanned in araster pattern over said first lens.
 7. The display of claim 1, whereinsaid scanning arrangement includes a rotatable faceted scanning wheelrotatable about a rotation axis, facets of said scanning wheel beinginclined at a plurality of different angles to said rotation axisthereof, and said scanning wheel being disposed on said beam path in amanner such that when said scanning wheel is rotated said light beam isscanned by said facets in mutually perpendicular axes in a rasterpattern over said first lens.
 8. The display of claim 1, wherein saidoriginal coherence radius is about equal to the radius of said lightbeam at said laser, said effective beam divergence is equal to aboutΘ_(A), where Θ_(A) is the acceptance angle of said projection optics,and said reduced coherence radius r_(c) has a value about equal toλ/Θ_(s), where λis the wavelength of said light beam and Θ_(s) is theangle subtended by said projection optics at said screen.
 9. The displayof claim 1, wherein said laser is one of first, second, and third lasersincluded in the display for generating and delivering respectively a redlight beam, a green light beam and a blue light beam.
 10. The display ofclaim 9, wherein said display further includes an optical arrangementfor combining said red, blue, and green, light beams along said beampath.
 11. The display of claim 9, wherein said laser is an opticallypumped semiconductor laser (OPS-laser).
 12. The display of claim 11,wherein said light beam delivered by said OPS-laser is delivered in aplurality of transverse modes.
 13. The display of claim 12, wherein saidlight beam has a divergence M² between about 2 and
 20. 14. The displayof claim 1, wherein said screen includes a first transparent sheethaving a plurality of raised features distributed over a surfacethereof, each of said features having a dimension greater than saidreduced coherence radius of said light beam, but less than a dimensionthat would be resolvable by a normal human eye at a normal viewingdistance from said screen.
 15. The display of claim 14, wherein saidsurface features include a first parallel array of cylindricalmicrolenses.
 16. The display of claim 15, wherein said surface featuresfurther include a second parallel array of cylindrical microlenses on anopposite surface of said transparent sheet, said second parallel arrayof cylindrical microlenses being oriented perpendicular to said firstparallel array of cylindrical microlenses.
 17. The display of claim 15,further includes a second transparent sheet, and wherein said surfacefeatures further include a second parallel array of cylindricalmicrolenses on a surface of said second transparent sheet, said secondparallel array of cylindrical microlenses being oriented perpendicularto said first array of cylindrical microlenses.
 18. The display of claim14, wherein said surface features include a regular two-dimensionalarray of convex spherical microlenses.
 19. The display of claim 18,wherein said microlenses are contiguous.
 20. The display of claim 18,wherein said microlenses are spaced apart and are separated by adistance greater than said reduced coherence radius of said light beam,but less than a dimension that would be resolvable by a normal human eyeat a normal viewing distance from said screen.
 21. The display of claim1, wherein said screen includes a transparent cell containing atransparent fluid having particles therein, said particles having adimension sufficiently small to scatter light.
 22. The display of claim21, wherein said particles have a dimension less than about 10micrometers.
 23. The display of claim 21, wherein said transparent cellincludes an agitating arrangement operable for causing said particles tobe in random motion with each other.
 24. The display of claim 23,wherein said agitating arrangement includes at least one rotatable bladelocated in said cell.
 25. The display of claim 24, wherein saidagitating arrangement includes at least a plurality of said rotatableblades located along a periphery of said cell.
 26. The display of claim25, wherein each of said blades is rotated magnetically by a drive adrive unit located outside of said cell.
 27. The display of claim 21,wherein said particles include particles of one of silicon dioxide,aluminum oxide, and titanium oxide.
 28. The display of claim 21, whereinsaid transparent fluid is water.
 29. The display of claim 21, whereinsaid cell has an RF transducer therein operable for discouraging saidparticles from coagulating.
 30. The display of claim 1, wherein saidscreen includes a transparent cell containing a transparent fluid havingparticles therein, said particles having a dimension sufficiently smallto scatter light and sufficiently small that said particles aresuspended in said fluid in Brownian motion with respect to each other.31. The display of claim 30, wherein said particles have a dimensionless than about one micrometer.
 32. A projection display, comprising: atleast one laser for delivering a light beam, said light beam of havingan original beam divergence and an original coherence radius, andpropagating along a beam path; a beam homogenizer; a first lens; ascanning arrangement for scanning said light in beam in a particularpattern over said first lens, such that, averaged over a time less thanabout the response time of a human eye, said light beam has an effectivebeam divergence greater than said original divergence thereof, saidscanned beam being delivered from said first lens into said beamhomogenizer to be homogenized thereby; a spatial light modulator, saidspatial light modulator arranged to receive said scanned light beamhomogenized by said beam homogenizer and spatially modulate saidhomogenized scanned light beam in accordance with a component of animage to be displayed; a screen for displaying said image; projectionoptics for projecting said homogenized scanned light beam onto saidscreen, said homogenized scanned light beam at said screen having areduced coherence radius less than said original coherence radiusthereof as a result of said scanning thereof; and wherein, said screenincludes a first transparent sheet having a plurality of raised featuresdistributed over a surface thereof, each of said features having adimension greater than said reduced coherence radius of said light beam,but less than a dimension that would be resolvable by a normal human eyeat a normal viewing distance from said screen.
 33. The display of claim32, wherein said surface features include a first parallel array ofcylindrical microlenses.
 34. The display of claim 33, wherein saidsurface features further include a second parallel array of cylindricalmicrolenses on an opposite surface of said transparent sheet, saidsecond parallel array of cylindrical microlenses being orientedperpendicular to said first parallel array of cylindrical microlenses.35. The display of claim 33, further includes a second transparentsheet, and wherein said surface features further include a secondparallel array of cylindrical microlenses on a surface of said secondtransparent sheet, said second parallel array of cylindrical microlensesbeing oriented perpendicular to said first array of cylindricalmicrolenses.
 36. The display of claim 32, wherein said surface featuresinclude a regular two-dimensional array of convex spherical microlenses.37. The display of claim 36, wherein said microlenses are contiguous.38. The display of claim 36, wherein said microlenses are spaced apartby a distance greater than said reduced coherence radius of said lightbeam, but less than a dimension that would be resolvable by a normalhuman eye at a normal viewing distance from said screen.
 39. Aprojection display, comprising: at least one laser for delivering alight beam, said light beam of having an original beam divergence and anoriginal coherence radius, and propagating along a beam path; anarrangement for homogenizing said light beam delivered by said laser; aspatial light modulator, said spatial light modulator arranged toreceive said homogenized light beam and spatially modulate saidhomogenized light beam in accordance with a component of an image to bedisplayed; a screen for displaying said image, said screen includes atransparent cell containing a transparent fluid having particlestherein, said particles having a dimension sufficiently small to scatterlight and wherein said cell has an RF transducer therein operable fordiscouraging said particles from coagulating; and projection optics forprojecting said homogenized light beam onto said screen.
 40. A screenfor a projection display, comprising: a transparent cell containing atransparent fluid having particles therein, said particles having adimension sufficiently small to scatter light and wherein said cell hasan RF transducer therein operable for discouraging said particles fromcoagulating.